Find some cool software that you want to use which just happens to be written in Go, then pick a simple looking ticket from the issue tracker and try to compete it.

If you like open source hardware, might I suggest buying or building a RuuviTag and digging into https://github.com/Scrin/RuuviBridge (e.g. issue #10).

Just jumping right in is very fast, you can see examples and start by making small changes. The changes that give you compiler or runtime errors are the things you need to look up, the rest is probably close enough to C, PHP, or whatever you're used to that you don't need to spend time reading about it.